A new plasma sialoglycoprotein, sgp120, initially identified by its ability to bind to the activated fourth component (C4b) of the classical complement pathway (CCP) was fully sequenced. The full-length cDNA for sgp120 was derived from human fetal liver cDNA libraries by standard cloning and sequencing technologies. We identified several forms of the gene that are present as a result of alternative splicing. Data support the idea that the C4b-binding form of the protein regulates the activation of the CCP. Sgp120 is also cleaved by kallikrein of the intrinsic coagulation-kinin generating pathway to produce protein fragments that may possess biological activity. Pigtryin (PT), an identified acute phase protein and postulated analog of human inter-alpha-trypsin inhibitor (IATI) was shown to have significantly more homology to sgp120. Thus, PT is the pig analog of human sgp120 and not IATI. Using a sensitive ELISA for sgp120 we have identified 2-3 fold increases in the serum sgp120 levels of burn patients indicating that sgp120 is also an acute phase reactant. Sgp120 shows strong homology to each of the three mature heavy chains of IATI. Homology to HC3 was most significant.